Erratic constant speed

I've got an IO360-A3B6D turning a Hartzell HC2YCR with a MT govenor. They all have about 680 hours on them. On take off the RPM has come down from 2700 to 2550 for the past 20 some odd hours. I tried to turn it up with the set screw but that didn't fix it. It made it worse. I did some tuch and goes the first take off it went to 2830 so reducing to climb out power I brought it back to 25 squared. When turning down wind I had over climbed to high and pushed the nose down to get back to altitude it over revved again. I took the govenor and prop to the prop shop and they said nothing was wrong with them. They didn't take them apart just tested them. I put them back on the engine and did a ground run. Now the RPM goes to 2720 on the ground with 29.6 inches of MAP. I brought it back and did a compression check. The lowest cylinder #2 is 77/80 at ambient temperature. The CHT on that cylinder is the lowest at 295 vs #4 at 318 being the highest. The EGT on that cylinder is the lowest at 1195 and the highest being 1325 on #4. The prop shop says the problem is in the engine. I don't see it. Any body have any ideas? Or should I move to a different prop shop?

Send the govenor to:

http://www.qaa.com/aircraft-prop-governors-and-accumulators

It sounds like the shaft in the govenor has worn causing the speed to fluctuate. I used QAA to repair a govenor and they had it done and back to me within 3 days.

TJ

Second opinion from a different shop?

A friend of mine had a similar issue with his Bonanza. He bought a new governor (his wife insisted) and the problem disappeared. It can't hurt to at least ask around.

Are you sure it isn't the tach? It might be worth the $30-ish it takes to get an optical tach from a hobby shop.

Here's a good place to start. http://www.avweb.com/news/maint/propeller_governor_diagnostics_195565-1.html
I would do the air pressure test to verify you don't have an oil transfer issue due to a worn bearing.

Yeah it's not the tach, I checked it with a tach checker and it is dead on. I can also hear the RPM change as I climb and dive consistent with the tach.